Generally, the next-generation mobile communication and wireless communication systems increasingly require an improved data transfer rate and an improved system capacity in a multi-cell environment. Due to the above-mentioned demand, technologies of a multi-input multi-output (MIMO) system for transmitting data using a plurality of antennas have been intensively researched by many developers and companies. A closed-loop MIMO system, which uses channel information to increase a data transfer rate in a multi-cell environment, can improve a transmission throughput or performance using this channel information.
Generally, in the MIMO system, a terminal, a mobile station (MS), or a user equipment (UE) is able to recognize information of a reception (Rx) channel using received data, but a base station is unable to recognize this channel information. Therefore, in order to improve the system performance or throughput using the above channel information, the base station must recognize this channel information. In case of using a time division duplex (TDD) scheme, uplink/downlink (UL/DL) channels between the base station and the mobile station are equal to each other, such that the base station is able to recognize channel information.
In the closed-loop MIMO system, the base station transmits data using information of a transmission (Tx) channel of each mobile station. In this case, base stations are unable to recognize information of the transmission (Tx) channel of the serving mobile stations, such that they receive channel information (e.g., a channel quality indicator (CQI)) from each mobile station through a feedback path.
The mobile station estimates a channel receiving data using a signal received from the base station. The mobile station calculates a CQI using the estimated channel, such that it can apply a modulation coding scheme (MCS) appropriate for a channel status when the base station transmits data. In addition, the mobile station selects a channel coefficient (e.g., a precoding vector of a codebook), which is the most appropriate for a channel status, from a known codebook using the estimated channel and the codebook. Channel information, which has been calculated by the mobile station using the estimated channel, is transferred over a feedback channel between the base station and the mobile station. The base station transmits data to the mobile station using the channel information, the selected MCS, and the codebook's precoding vector which have been received from the mobile station.
Presently, many researches for the closed-loop MIMO system have been intensively conducted, and the closed-loop MIMO scheme is being applied to not only a single-cell environment but also a multi-cell environment. However, researched for reducing influences caused by interferences from other cells in multi-cell environment, by which usage of precoding matrixes in a codebook used in each cell is restricted, have been barely performed.
In the closed-loop MIMO system in a multi-cell environment, individual base stations allocate an optimum pre-codebook to each mobile station on the basis of a channel status between each base station and each mobile station, such that the base stations are able to provide each mobile station contained in a cell with a high-quality service.
However, because the strength of a signal transmitted from a base station of a cell is weak when a mobile station located at an edge of the cell receives the signal, the signal is affected by another signal which is processed with a optimum precoding matrix allocated to another mobile station in a neighboring cell and then transmitted from a neighboring base station located in the neighboring cell. Due to the inter-cell interference caused by the signal from the neighboring cell processed with the optimum precoding matrix, performance or throughput of a mobile station located at a cell edge is unavoidably decreased.